This invention relates to power switching circuits. More specifically, it relates to a power inverter having a clamped neutral point and which provides an output having reduced harmonic content.
Conventional inverters provide a pair of power transistors connected in series between positive and negative DC terminals which operate on opposite half cycles to produce an AC output. These power inverters, however, have outputs with high harmonic content and further do not function well with reactive loads. Under such load conditions, reactive currents at the output result in a nonideal output voltage waveform having increased harmonic content.
An attempt to reduce the harmonic content of the output of a power inverter is illustrated in U.S. Pat. No. 4,443,841 (hereinafter Mikami). In Mikami, each AC phase output terminal is clamped to a neutral terminal in a DC power source by a clamping circuit comprised of two transistors. One of the transistors in the clamping circuit is enabled during a positive cycle of inverter operation as well as for a short time therebefore and thereafter. The other transistor in the clamping circuit is enabled during the negative cycle of inverter operation as well as for a short time therebefore and thereafter. In this way, the potential of each AC phase output terminal is never permitted to change from that of a positive terminal in the DC power source directly to that of a negative terminal in the DC power source, or vice versa. Mikami, however, does not respond to reactive currents which can be generated as a result of reactive loads.
An inverter having an output waveform with low distortion and low harmonic content particularly for use with reactive loads, therefore, is needed.